Gas chromatographs are used to measure the quantities of various chemicals in a mixture. A sample of the mixture is injected in gaseous form into a stream of carrier gas as it is about to enter a chromatographic column. The various chemicals take respectively different amounts of time to pass through the column so that they appear as sequential concentrations in the stream of carrier gas flowing out of the column. A detector is coupled to the output of the column so as to provide an output signal having a value known as a "baseline value" when carrier gas is emerging from the column and a value that changes with the degree of concentration of each sample gas as it emerges from the column so as to form peaks. By integrating the area between each peak and the baseline, the amount of the corresponding chemical in the injected sample can be determined.
Thermal conductivity detectors are widely used to provide the output signal referred to. In their simplest form, they are comprised of a cell having an electrically heated filament suspended in a cavity. As the output from the column flows through the cavity, the rate at which heat flows from the filament to the wall of the cavity varies with the thermal conductivities of the gases in the cavity. The thermal conductivity of the carrier gas differs from the thermal conductivities of the sample gases, and the thermal conductivities of the sample gases mixed with carrier gas vary with the concentration of the sample gas in the carrier gas. Means are provided for deriving a signal that varies with the rate of heat flow. Accordingly, an output signal of the cell has a baseline value when carrier gas is flowing through its cavity and peaks when the concentrations of the respective sample gases are flowing through the cavity. The output signal is the voltage required to keep the filament at a constant temperature.
One of the problems encountered with such detectors is that the heat flow between the filament and the wall of the cell is directly affected by the temperature of the wall. For this reason it has been customary to reduce the effect of ambient temperatures on the temperature of the wall by imbedding the cell in a large block of aluminum. Even better results are achieved by imbedding two cells in the block, passing carrier gas through one, passing the elutant from the column through the other, and connecting the filaments in a bridge circuit. Thus, if an ambient temperature change causes the wall of each cell to vary in like manner, its effect will be balanced out by the bridge. Either construction is expensive and requires several hours after the application of power to the filament to achieve sufficient thermal equilibrium for accurate readings.
In our U.S. patent application, Ser. No. 730,559, filed on Oct. 7, 1976, and entitled "Modulated Fluid Detector", a single cell thermal conductivity detector is described that is inexpensive and capable of attaining accurate readings within minutes after power is applied to the filament. The cavity of the cell is switched at a given frequency from the output of the column to a source of reference gas so that its output signal, i.e., the voltage required to keep the filament at a given temperature, varies between a value determined by the thermal conductivity of the gas eluting from the column and a value determined by the thermal conductivity of the reference gas. The switching frequency is such that a number of switching cycles occur during the elution of each peak of sample gas from the column. The alternating voltage V thus produced is AC coupled to a synchronous detector wherein it is mixed with a control voltage that is in phase with the switching of the cavity of the cell from the column to the source of reference gas. In this detector, the temperature of the wall of the cell can be permitted to follow the ambient temperature because it changes so slowly with respect to the switching frequency as to have the same effect on the amplitude of the output signal of the cell whether gas from the column or reference gas is flowing through the cell cavity. The synchronous detector derives a signal proportional to the difference between the level of the output signal of the cell under these two conditions so that the effect of variation in the temperature of the cell wall is eliminated. Thus, instead of imbedding the cell in a block of metal, it can be imbedded in a small inexpensive wafer of ceramic material. Furthermore, accurate readings can be attained within a matter of minutes because it is not necessary to wait for thermal equilibrium to be established.